The present invention relates to web-offset printing presses. Specifically, the present invention relates to a web-offset printing press for simultaneously printing on two webs of paper and allowing an on-the-fly change of plate cylinders.
In a web-offset printing press, an image is transferred to a web of paper by a blanket cylinder, which in turn receives the image from a printing plate cylinder, or simply plate cylinder. In many presses, plate cylinder/blanket cylinder pairs (referred to as xe2x80x98couplesxe2x80x99), are employed on opposite sides of a traveling web to print an image on both sides of the web simultaneously. In addition, multiple couples are often used on a single web side to achieve multiple-colored images. In many cases, for example in presses used to print newspapers, it is desirable to achieve a so-called 4/1 printing job, in which four couples are used to print a four-color image on one side of a web, while a single couple is used to print a one-color image (e.g. a black image) on the opposite side of the web.
A printing press capable of printing two 4/1 print jobs simultaneously is described in U.S. Pat. No. 6,145,435, filed Nov. 20, 1997 by John Richards entitled WEB-ROTARY PRINTING PRESS AND METHOD FOR SIMULTANEOUSLY PRINTING TWO WEBS, expressly incorporated herein by reference. According to that patent by the same inventor of the present application, a series of printing units (each unit including a pair of opposing couples) prints a four-color image on single sides of two webs which travel back-to-back through the press. The two webs are then separated, and additional units print a single-color image on the opposite side of each web.
One shortcoming of this and other printing presses is created by the fact that the presses must be stopped to accommodate, for example, edition changes of the newspaper. Edition changes require a different image to be placed on the web, but typically changes only occur for a single image on the first page. Accordingly, only a single plate cylinder, for example the plate cylinder printing a black image of a four-color image, needs to be changed for each web traveling through the press. Despite the fact that only one cylinder (for each web) needs to be changed, the entire conventional press must be halted in order to install the new plate cylinder.
French Patent Application No. 2 377 888 describes a printing press in which a web 8 is printed with multicolor images on both sides, is then printed with varnish or resin film and is afterwards superimposed with a synthetic web 9 and passed through a nip in order to obtain a printed image with a very high brilliance. According to FIG. 3 of this application, the two webs 8, 9 are printed on their respective sides and are afterwards either wound up on a bobbin or are fed to a group of cylinders where they are glued together in order to provide for a print carrier or support which is printed on its two outer faces. The whole print carrier is wound on the bobbin.
U.S. Pat. No. 4,412,490 discloses a paper web guiding mechanism. This patent does not teach or disclose passing two webs with their unprinted sides back-to-back through one or more blanket-to-blanket and then printing on the their unprinted sides.
A web-offset printing press according to the present invention includes, for example, three initial printing units for printing color images on outer sides of a pair of webs traveling through the press. The webs travel, for example, back-to-back through the press so that the outer side of each web receives the three images.
The press further includes, for example, three additional printing units. The first additional unit includes, for example, two couples, each of which can apply a fourth color image (e.g. a black image) to the outer sides of the two webs.
The next additional printing unit includes, for example, two couples. The webs may be separated before passing through this unit, so that only a single web passes through this unit. The first couple of this unit prints, for example, a single-color image on the inner side of one of the webs. The other couple of this unit prints, for example, the fourth color image to the outer side of that same web.
The third additional printing unit performs, for example, similar functions for the second of the two webs. This unit therefore includes, for example, two couples, one printing a single-color image on the inner side of the second web and the other printing the fourth color image to the outer side of the second web.
Because the exemplary press of the present invention includes two separate couples capable of printing the fourth color image for each web, the plate cylinder of one of these couples may be changed while the other is still printing. Thus one plate may be replaced, while still printing the entire 4/1 image on two webs simultaneously, without stopping the press.